This invention is not the result of federally sponsored research or development, and no government license rights exist as of the time of filing herein.
1. Field of the Invention
The present invention relates to artificial lift for hydrocarbon wells. More particularly, the invention relates to an improved housing for a production pressure operated gas lift valve.
2. Background of the Related Art
The production of fluid hydrocarbons from wells involves technologies that vary depending upon the characteristics of the well. While some wells are capable of producing under naturally induced reservoir pressures, more common are wells which employ some form of an artificial lift production procedure, During the life of any producing well, the natural reservoir pressure decreases as gases and liquids are removed from the formation. As the natural formation pressure of a well decreases, the hydrostatic pressure from fluid within the production tubing becomes greater than the formation pressure, thereby inhibiting the flow of hydrocarbons from the formation to the surface. This phenomenon may also occur naturally in deep wells that encounter flow resistance from the substantial hydrostatic head.
In such wells, it is conventional to periodically remove the accumulated liquids by artificial lift techniques. One such technique which has been know for many years involves the use of gas lift devices.
Gas lift is a method of producing hydrocarbons by which gas is injected through a pressure-sensitive valve into the tubing. One or more valves are placed at or above the production zone. In operation, gas under pressure is injected into the annular space between casing and tubing above the production packer. The pressurized gas is delivered from the gas lift valve and into the tubing. Fluid that is in the tubing above the gas injection port is displaced, lightened by mixing with the gas, and is raised to the surface by the expanding gas.
The gas lift process closely simulates the natural flow process but provides a highly economical enhancement of that process. When natural gas is produced with oil or is available from nearby wells from injection, gas lift becomes an economical means for enhancing the hydrocarbon recovery from an oil well.
Some gas lift valves are tubing-retrievable, meaning they are placed between joints of the tubing string and are pulled along with the tubing. Other gas lift valves are wireline retrievable. Such valves are run in side pocket mandrels and pulled and replaced by means of a wire line unit. Wireline retrievable gas lift valves are typically configured between joints of the tubing string.
Over the years, gas lift valves have been designed which operate based upon different pressure sources. One common valve is the production-pressure operated (PPO) gas lift valve. In this arrangement, pressure from inside of the tubing provides the primary pressure source for operation of the gas lift valve. Hydrostatic pressure of fluid within the tubing, coupled also with pressure from the producing formation causes fluids from the tubing to enter the pressure chamber within the gas lift valve. At the same time, pressure from gas injected into the tubing-casing annulus is also forced into the pressure chamber via a separate through-opening. Together, these fluids act upon a bellows within the pressure chamber, above a ball and seat valve.
The bellows is spring-biased or gas-charged to hold the pressure chamber valve in a closed position. However, when a preset level of pressure is reached, the bellows contracts, lifting the valve stem and ball off the seat. Fluids acting upon the bellows are then expelled from the gas lift valve into the tubing. In this manner, the hydrostatic head within the tubing is lightened.
The typical seat for a production pressure operated gas lift valve resides on a housing known as a cross-over housing. In this embodiment, production fluid and casing gas both enter the pressure chamber of the gas lift valve through the cross-over housing. The production fluid and the casing gas cross paths through the housing, but do not commingle within the housing; hence, the name. Production fluids enter the cross-over housing via a series of radial apertures, or jets, machined longitudinally into the housing. Casing gas enters the housing via one or more elbow-shaped through- openings which places the annulus and the seat of the cross-over housing in direct fluid communication. In this manner, formation fluids apply pressure on the bellows, while casing gas acts directly on the seat under the ball of the valve.
At some preset point, the combined pressure from the formation fluids and the casing gas will unseat the pressure chamber valve. When this occurs, the formation fluid commingles with the injected gas from the casing within the pressure chamber. When the production pressure overcomes the preset charge or spring force of the bellows assembly, the bellows is compressed and the valve stem and ball is lifted off the valve seat, opening the pressure chamber valve. Because the casing gas is maintained at a pressure greater than that of the formation, the formation fluid is expelled back through the cross-over housing jets. This means that formation fluids, commingled with casing gas, make a 180 degree turn, exiting the pressure chamber through the jets. The pressure on the bellows within the pressure chamber then drops, causing the valve to reseat.
It has been discovered that an operational problem sometimes arises with respect to the reseating of the pressure chamber valve. In some instances, the bellows is unable to recognize a pressure drop within the pressure chamber after the valve is unseated. Analysis of this phenomenon reveals that the configuration of the jets sometimes restricts the ability of the tubing pressure to be sensed above the cross-over housing. In this regard, sonic flow, or critical flow, is created within the crossover configuration of the housing such that the pressure on the bellows remains at a level sufficient to the keep the pressure chamber valve unseated. This, in turn, causes continuous injection of gas into the production string, thereby inhibiting hydrocarbon production.
It is, therefore, an object of the present invention to provide a gas lift valve wherein the pressure chamber valve closes properly after being unseated, thereby injecting gas into the production string intermittently.
It is a further object of the present invention to provide a configuration for a cross-over housing within a production pressure operated gas lift valve which facilitates the egress of casing gas from the pressure chamber after the pressure chamber valve has been unseated.
Yet another object of the present invention is to replace the series of radial apertures within the seat housing of a production pressure operated gas lift valve with a substantially continuous through-opening.
Still further, an object of the present invention is to provide a substantially continuous aperture within the cross-over housing for a production pressure operated gas lift valve, whereby the substantially continuous aperture permits an increased volume of gas to flow through the cross-over housing without reaching critical flow so that the bellows can sense a pressure drop, thus allowing the pressure chamber valve to be reseated.
And another object of the present invention is to provide a more efficient production pressure operated gas lift valve having an improved cross-over housing capable of being utilized in both top and bottom latch gas lift valves.
Finally, an object of the present invention is to provide a cross-over housing for a gas lift valve which is easier to machine and more economical to produce.
The present invention provides a more efficient gas lift valve by presenting an improved cross-over housing. In the present invention, the series of radial apertures, or jets, typically utilized within the cross-over housing of a production pressure operated gas lift valve are removed. In their place is a substantially continuous, arcuate aperture. The aperture will also have an area significantly greater than the area of the casing gas through-opening, or seat. This allows the bellows within the pressure chamber of the gas lift valve to sense the eventual pressure drop of tubing pressure which occurs during gas injection. This, in turn, allows the pressure chamber valve to be reseated.